The viscosities ͑͒, conductivities ͑͒, and enthalpies of dissolution (⌬H sln 0 ) of LiCF 3 SO 3 ͑LiTF͒ and LiN(CF 3 SO 2 ) 2 ͑LiTFSI͒ are measured in ␥-butyrolactone ͑BL͒, ␥-valerolactone ͑VL͒, and mixtures of ethylene carbonate ͑EC͒ with tetrahydrofuran ͑THF͒ and diglyme ͑DG͒: EC:THF ͑20:80, in moles͒ and EC: DG ͑45:55, in moles͒. From the variations of with the salt concentration, the B and D coefficients in the extended Jones-Dole equation are determined at different temperatures. The concentration dependence of the conductivity is analyzed on the basis of a model involving ͑i͒ a chemical equilibrium between free ions and ion pairs, ͑ii͒ the use of the Walden product to correct molar conductivities for viscosity variations, and ͑iii͒ the calculation of ion activity coefficients by the cube root law derived from the quasi-lattice model. According to this model, the limiting molar conductivities (⌳ ϱ ) and ion pair dissociation constants (K D ) are inferred. The solvation enthalpies of the salts are deduced from the heat of dissolution of LiTF and LiTFSI in BL, VL, and the EC:DG mixture. The formation of solvent-separated ion pairs in lactones and contact ion pairs in the EC mixture is deduced from the analysis of the experimental data as the result of a competition between ion-ion and ion-solvent interactions. The dissociation coefficient of ion pairs determined by Raman spectroscopy agrees well with those deduced from conductivity measurements. From the variation of the area of the band at 676 cm Ϫ1 with salt concentration, a coordination number of 4 molecules is found for the solvation of the Li ϩ ion by BL molecules.The ongoing rapid expansion of new portable electrical technologies in the past 15 years boosts the need for rechargeable batteries with higher densities of energy and power storage. The first commercial lithium ion secondary cell was introduced about 10 years ago by Sony. 1,2 The formulation of the electrolyte, liquid or gel, is of prime importance as it has to conform to many requirements.Liquid electrolytes for rechargeable lithium-ion batteries or electrochromic devices 3 and high power super capacitors 4 are usually composed of a solvent of high dielectric constant having a good passivating property such as ethylene carbonate ͑EC͒, mixed with low viscosity solvents such as linear alkyl carbonates or ethers like tetrahydrofuran ͑THF͒ or 2-methoxyethyl ether ͑diglyme, DG͒ in order to improve the conductivity. 5 Optimal composition of EC:ether based mixture is 45:55 for EC:DG, and, 20:80 for EC:THF ͑in moles͒. The results obtained with these solvent mixtures are compared with lactone-based solvents, namely, ␥-butyrolactone ͑BL͒ and ␥-valerolactone ͑VL͒. These two dipolar aprotic solvents are stable, they have a high boiling point, a high permittivity, and a moderate viscosity. 6 Lithium salts like LiCF 3 SO 3 ͑lithium triflate, LiTF͒ and Li(CF 3 SO 2 ) 2 N ͑lithium trifluorosulfonimide, LiTFSI͒ offer good stability in the solid state and in solution, and good safety characteristics. 7,8 Due to ...
